Light-Emitting Fluorene Photoreactive Liquid Crystals for Organic Electroluminescence

Abstract

Light-emitting liquid crystals for organic light-emitting diodes (OLEDs) require low-temperature liquid crystal phases for room-temperature processing and a range of molecular energies for electron and hole injection, as well as tunable color and color purity for multicolor OLEDs. We report a number of light-emitting polymerizable liquid crystals (reactive mesogens) based on 2,7-disubstituted-9,9-dialkylfluorene, whose energy levels can be tuned for optimized charge injection and light emission. As a consequence of these systematic property/structure investigations small molecule reactive mesogens have been synthezised, which exhibit low melting points, even below room temperature and nematic phases above room temperature as single components. Many of the molecules retain a supercooled nematic phase on cooling to room temperature. Simple binary eutectic mixtures of reactive mesogens with identical aromatic cores form light-emitting nematic phases at room temperature with a high clearing point to generate a high order parameter. The ionization potential of six-ring fluorene reactive mesogens can be tuned between 4.93 and 5.57 eV by chemical modification of the aromatic cores. Similarly the emission spectrum can be tuned from blue to green. A typical performance for an OLED using such liquid crystalline materials as a cross-linked polymer network is described.

New Photovoltaic Concept: Liquid‐Crystal Solar Cells Using a Nematic Gel Template

“…[16,17] Polymerizable LCs, with two polymerizable groups attached through flexible aliphatic spacers to an aromatic core, are attracting interest as organic semiconductors for electroluminescence [18][19][20][21] and organic transistors. [22,23] Their advantages include spontaneous self-assembly, relatively high charge mobility (1 × 10 -3 cm 2 V -1 s -1 ) [20] , easy deposition by spin-coating or ink-jet printing and lithographic photopatterning [18] to form multilayer polymer networks.…”

“…The control device does not require a specific electronblocking layer since it is completely insoluble and not washed. [21] Figure 4a also shows the current-voltage characteristics of the control device with the same irradiation conditions as the gel device. The control device shows an external quantum efficiency over five times smaller than the gel device, which confirms that the gel-induced distributed interface improves efficiency considerably.…”

“…We have previously reported that a fully crosslinked film of a homologue of 1 is completely insoluble. [21] The gel may allow partial intermixing of the overlying layer, thus increasing the interaction depth. A detailed look at the spatial frequencies of the bilayer and gel interfaces, as approximated by the crosslinked and washed gel surfaces illustrated in Figure 3a and b, may also explain the improved performance of the gel device.…”

Photocrosslinkable π-conjugated cruciform molecules for electronic/optoelectronic applications

“…This means that unreacted monomers should not polymerize spontaneously during the fabrication and operation of a device. Acrylates and methacrylates may also show a greater tendency to photochemical degradation during crosslinking process [2,14]. The reaction pathways of 1,4-pentadiene monomer to form poly(1,4-pentadienes) and acrylate monomer to form a relatively flexible polyacrylate backbone are as follows:…”

“…There are several reports describing the photochemical or thermal crosslinking of organic side chain polymers to form insoluble polymer networks as charge-transport and/or emission layers [1][2][3][4]. However, the photochemical crosslinking process often caused a substantial degree of photochemical degradation and a reduction of charge-carrier mobility.…”